It takes a Boeing 747 about 10 hours and 136,000 liters of fuel to fly from eastern China to Hawaii. This week, Andre Borschberg will attempt the same flight in a sun-powered aircraft that resembles a dragonfly. He’ll do it over five days, without a drop of fuel.

No one before has attempted a solar-powered flight over such a large expanse of ocean -– 8,160 kilometers. If bad weather or other problems force him to ditch his Solar Impulse plane, Borschberg will have only his wits and a life raft to save him.

Borschberg acknowledges the dangers. So does his fellow Swiss adventurer, Bertrand Piccard, who flew the single-seat plane to China in April and is slated to fly it from Hawaii to Phoenix later this month.

“Yes, we are nervous. I am nervous also,” Piccard said in an interview in Nanjing. “But more than anything, we are impressed. We’re in awe of the enormous distance over water that we have to do: Andre for the first part, and me for the second part.”

Piccard and Borschberg hope to be the first pilots to fly a solar-powered plane around the world. After 12 years of planning, networking and fundraising, they launched their tag-team expedition in March, flying from the United Arab Emirates to Oman, and then on to India, Myanmar, Chongqing, China, and Nanjing.

On Wednesday, scores of Chinese students filed into a portable hanger in Nanjing, a city of 8 million people, to meet the pilots and take a look at their odd-looking craft. The students gawked at the enormous wingspan of the Solar Impulse – wider than that of a 747.

Despite its size, the plane weighs only 2,300 kilograms – about as much as a minivan. Much of that weight comes from the four batteries that sit behind four propellers. When the plane is in flight, those batteries are recharged by 17,248 feather-light solar cells on top of the wings. The batteries then help power the aircraft at night.

Flying the plane is complex. To husband its power, the pilot takes the aircraft as high as possible during the late daylight hours. Then, in the darkness, the plane’s engines are turned off and the aircraft sails like a glider, dropping slowly for about three hours. Then the engines are turned back on, drawing on the batteries until daybreak.

“It is difficult to fly, especially at the beginning,” said Borschberg, a 62-year-old former fighter pilot with the Swiss air reserve.

With its lightness and wide wingspan, the plane reacts slowly to a change in controls, making it easy for a pilot to overcompensate, he said.

Some aviation experts have mocked the plane’s slowness; it has a top cruising speed of about 128km/hr. Piccard said such critics were missing the point. The goal of Solar Impulse isn’t to set speed records. It’s to demonstrate that a plane can fly around the world without a gas tank and, theoretically, with the sun’s rays, keep flying forever.

“We really want to prove that energy efficiency, solar power and modern technology can achieve the impossible,” Piccard said.

Piccard, 57, initially launched Solar Impulse in part to continue a family legacy of invention and exploration. In 1960, his father, Jacques Piccard, became the first person to reach the bottom of the Mariana Trench. He did it with partner Don Walsh in a submersible that his own father, physicist Auguste Piccard, had helped design.

Bertrand Piccard grew up around space program celebrities when he lived in Florida, where his father worked as a US Navy contractor. Visitors to the house, he said, included aerospace engineer Wernher von Braun, along with Neil Armstrong, Buzz Aldrin and other NASA astronauts.

Those encounters fanned his interest in air expeditions. Later the experience would lead him to master hang gliding, micro-light aircraft and hot-air balloons.

In search of solar power

His big break came in 1999, when he and co-pilot Brian Jones became the first to circle the globe non-stop in a hot-air balloon. For Piccard, the 19-day trip was a triumph but also a nail-biter. In the final hours, he and Jones nearly ran out of fuel, partly because their balloon had stalled over the Pacific, burning gas to stay aloft above the windless sea.

“It was then that I thought about the next big flight,” he recalled. “It has to be with no fuel.”

Piccard presented his initial concept of Solar Impulse to the Swiss Federal Institute of Technology, which chose Borschberg, an engineer, to examine it. Borschberg eventually became Piccard’s collaborator. The two assembled a design team and lined up sponsors and partners. Scores of companies, ranging from Omega to Google, have since contributed to Solar Impulse, either by donating materials or expertise.

One big challenge for the Solar Impulse pilots is the amount of time they must spend, individually, aloft in the unpressurised cockpit, with only infrequent 20-minute naps. Borschberg said the plane, with its lightness and vulnerability to wind changes, couldn’t be on autopilot for any longer than that.

To prepare for those lonely days aloft, the pilots practiced with a flight simulator, and they’ve developed mental exercises to help themselves stay alert.

Setting off as early as Tuesday

Borschberg said the first possible opening for him flying to Hawaii would be Tuesday, but if the ‘weather window’ isn’t good, he’ll wait for a better date.

Although Solar Impulse bills itself as a carbon-free aircraft, the entire operation can’t make that claim. Roughly 160 people are involved, both travelling with the plane and in Monaco, where a mission-control team is based. Support equipment and crew are flown on cargo planes, generating the same emissions as any other large aircraft.

Piccard makes no apologies for that. He said technology would advance only if it built on the backs of previous innovations.

“I am not someone who wants to go back in the caves with candles,” he said. “The psychology of human beings is not to go backward; it is to go forward. But we can go forward in a clean way.

“Never underestimate the power of technology development. We should embrace it.”

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